The primary goals are to support Projects 1, 2, 3, and 4 by supplying precise and accurate biochemical measurements of the calcium-regulating hormones and the markers of bone formation and bone resorption and to test the hypothesis that the efficiency of calcium absorption is a determinant o bone mass and its rate of loss. The relationship between bone mass and its rate of loss and the efficiency of calcium absorption will be established by measuring calcium absorption i women being studied pre- and post menopausally (Project 1), in adult twins (Project 2) , and in men and women over the age of 65 taking calcium or vitamin D supplements (Project 4), who are having regular measurements of bone mass. Calcium absorption will be assessed from the specific activity of radiocalcium in serum, 3 and 5 hours after a standardized breakfast containing 250 mg calcium labeled with radiocalcium. Precise and accurate assays for serum calcitriol, calcidiol, parathyroid hormone, osteocalcin, bone alkaline phosphatase, tartrate resistant acid phosphatase and for urine hydroxyproline have been established and will be made available to assist in testing the hypothesis being proposed in Projects 1, 2, 3, and 4. In addition, Core C will be responsible for ensuring that all biological samples arising from Projects 1, 2, 3, and 4 are logged and stored under optimal conditions, that assays are performed with precision and accuracy and that the data generated are transferred in a timely fashion to Core A analysis.